Method of preparing phosphine



NOvQ s, 1963 ORDON 3,109,785

, METHOD OF PREPARING PHOSPHINE Filed July 27, 1960 CATHODE PHOSPHORUS United States Patent 3,199,785 METHOD 6F PREPARING PHOSPHINE Irving Gordon, Niagara Falls, N.Y., assiguor to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York Filed July 27, 1960, Ser. No. 45,668 16 Claims. (Cl. 204-9tl) This invention relates to the preparation of phosphine by the electrolysis of phosphorus.

Heretofore, phosphine has been prepared by the reaction of metallic phosphides or phosphonium halides with water, and by the hydrolysis of elemental phosphorus. These methods have been unsatisfactory because of the high production costs and/or because the phosphine prodnot is in an impure form.

United States Patent No. 1,375,819, issued April 2 1921, to Henry Blumenberg, In, discloses a method for preparing arsine by the electrolysis of a salt or oxide of arsenic in the presence of sulfuric acid and potassium sulfate or other compounds capable of liberating nascent hydrogen upon electrolysis. However, phosphine is not produced under the conditions set forth by Blumenberg when an oxide or salt of phosphorus is employed.

W. R. Grove, in the Journal of the Chemical Society, vol. 16 (1863), pp. 263-272, discloses the use of an electric current to boil moist molten phosphorus and produce phosp-hine thereby. Such a technique requires a high voltage, and converts only a small amount of phosphorus to phosphine.

It is an object of this invention to provide a method of producing phosphine by electrolytic means.

Another object of the invention is to provide a more economical method of producing phosphine.

Another object of the invention is to provide an elec trolytic process for the simultaneous production of phos phine, a halogen gas, and an alkali metal hypophosphite.

These and other objects of the invention will be apparent from the following detailed description of the invention.

It has been discovered that phosphine, a halogen gas and an alkali metal hypochlorite can be prepared simultaneously by electrolytic means wherein an electric current is passed between an anode and a cathode in contact with an electrolyte comprised of an aqueous solution of an alkali metal halide, at least a portion of the cathode being in cont-act with molten phosphor-us, while maintaining the anode free from contact with molten phosphorus.

The accompanying drawing is a schematic illustration of a typical electrolytic cell suitable for carrying out the novel process.

Referring to the drawing there is shown cell vessel 10 having a cathode section 11 and an anode section 12, sections 11 and 12 being separated by a porous diaphragm 13. Gas tight cover 14 having ports 15, 16, 17,

18 and 19, is secured to the top of cell vessel 10.

Molten phosphonis 20 is contained in the bottom portion of cathode section 11, the upper level being indicated by interface 21. An electrolyte 22 comprised of an aqueous solution of an alkali metal halide is contained in the cathode section 11 and anode section 12, the upper level of the electrolyte being indicated by electrolyte interface 23. Cathode 24 extends through port and electrolyte 22 into the molten phosphorus 20. Cathode 24 is shown here as a solid plate, but other forms of cathode can be employed, as will be discussed more fully hereinafter.

'Anode 25 extends through port 16 into the electrolyte 22 contained in anode section 12. Electrolytic conductors 26 and 27 connect the anode and cathode, respectively, to the positive and negative poles, respectively, of a source of electrical energy 23.

When an electric current is impressed upon the system a phosphine-containing gas is generated in the cathode section 11 and is discharged through catholyte gas discharge port 29, which extends through port 17. At the same time, the halogen gas or the anolyte gas formed in the anode section 12 is discharged through anolyte gas discharge line 31, which extends through port 18. The aqueous alkali metal hypophosphite produced in cathode sec tion 22 may be withdrawn through catholyte outlet line 34, provided :with valve means 35.

A fresh supply of molten phosphorus and/or electrolyte may be introduced into cathode section 11 by means of tunnel 32 which passes from the cell vessel exterior through port 19 into cathode section 11. If desired, a motor driven impeller 33, or other suitable agitation means, may be positioned in the bottom portion of cathode sec-tion 11 to efifect agitation of the molten phosphorus.

It will be recognized by those skilled in the art that the design of the electrolytic cell shown in the drawing may be modified Without departing from the spirit of the invention. For example, the cathode is shown in the drawing as a solid plate, but a liquid cathode such as mercury may be employed. In such a case, liquid mercury is placed in the bottom of cathode section 11 below the molten phosphorus, and an electrical conductor 27 is extended through port 15 into the liquid cathode at the bottom of cathode section 11. In this case, it is necessary to agitate both the liquid cathode and the molten phosphorus in order that the liquid cathode contacts not only the molten phosphorus, but also the electrolyte, thereby permitting the current to pass between the cathode and anode.

The cell vessel may be constructed of any impervious material such as glass, ceramics, rubber-lined steel and the like.

Diaphragm 13 may be constructed of any suitable porous material such as sintered glass, porous alundum, ionexchange membranes, plastic cloth, glass cloth and the like.

Any material having a hydrogen overvoltage as normally measured in the absence of phosphorus exceeding the hydrogen overvoltage of smooth platinum may be employed as the cathode. Typical cathodic materials include lead, lead-mercury amalgam, tin, mercury, cadmium, copper, bismuth, aluminum, zinc, brass, silver, nickel, tellurium, Monel, gold, and alloys thereof. For example, the alloy known as Woods metal, which is an alloy containing fifty percent bismuth, twenty-live percent lead, twelve and one-half percent tin, and twelve and one-half percent cadmium, may be employed. This alloy may be used in either liquid or solid form. Black phosphorus may also be employed as a cathode material.

It is desirable to employ a cathode in a form having a high unit of area per unit of weight. As indicated previously, the drawing shows the cathode in the form of a solid plate. If desired, when a solid cathode is employed, the cathode may have the form of a helical coil, wire gauze or screen, perforated sheets, and the like.

The anode is preferably constructed of graphite, but any material capable of resisting anionic corrosion under the electrolysis conditions obtained may be employed. For example, noble metals such as platinum may be employed if desired.

An aqueous solution of an alkali metal halide is employed as the electrolyte. Typical examples of suitable alkali metal halides include sodium chloride, potassium chloride, lithium chloride, sodium bromide, sodium iodide, potassium bromide, potassium fluoride, potassium iodide, lithium bromide, lithium iodide, and mixtures thereof. The initial concentration of alkali metal halide in the solution will vary with the solubility characteris- V tics of the alkali metal halide, but is preferably between about ten percent by'weight and the saturation concentration. Alkali metal chlorides are preferably employed boiling thereof. For this reason, the temperature of the molten phosphorus and electrolyte is maintained within the range between about forty-four degrees and about two hundred and eighty degrees centigrade, and preferably between about fifty and about one hundred and twenty degrees centigrade. Temperature control of the phosphorus and the electrolyte may be readily obtained by means of constant temperature bath (not shown in the drawing), surrounding cell vessel 10, but any suitable temperature control means may be employed. For example, on start-up of the electrolytic process, the molten phosphorus and electrolyte may be heated to a temperature within the aforesaid temperature range by means of an external source of heat, and maintained at this temperature by means of a constant temperature bath, and/ or the heat generated in the cell during electrolysis.

The rate of phosphine production and the purity of the phosphine product varies with the current and current density. When a low current density is employed, a gaseous mixture of phosphine and hydrogen is produced at the cathode, the resulting gas mixture containing a high concentration of phosphine. However, under these conditions, the production rate of the gas mixture is relatively low, being generally below the level which is considered economically feasible. When high current densities are employed the production rate of the phosphine-containing gas is increased, but the concentration of phosphine is reduced. Increasing the cathodic current density will increase the production rate, but also reduces the phosphine concentration in the catholyte gas. It will be recognized by those skilled in the art that the optimum current and optimum current density will vary with the size and design of the cell, but in each case it is important to employ those conditions that yield a catholyte gas having a high concentration of phosphine consistent with commercially feasible production rates. For example, a cathodic current density within the range between about five and about one thousand amperes per square foot yield optimum results in most instances, but higher or lower densities may be employed if desired. It is important to employ in each case, current which will produce a voltage drop across the system of less than about twenty volts and preferably less than about ten volts. When the voltage drop is in excess of about twenty volts, a significant proportion of the electrical energy imparted to the cell is wasted in heating the ingredients in the cell, instead of being utilized to effect electrolytic decomposition thereof.

The catholyte gas produced in accordance with the instant novel process is a mixture of phosphine and hydrogen, containing as high as ninety percent phosphine or higher.

The anolyte gas is a halogen gas, such as chlorine, bromine, iodine corresponding to the halide component of the alkali metal halide employed in the electrolyte.

Alkali metal hydrophosphite forms in the catholyte as electrolysis progresses, and may be recovered by withdrawing a portion of the catholyte continuously or batchwise from the cell, while adding a corresponding volume of fresh electrolyte.

The following examples are presented to define the invention more clearly without any intention of being limited thereby. All parts and percentages are by weight unless otherwise specified.

4 Example 1 An electrolytic cell was constructed as follows: A five hundred milliliter glass beaker having a gas-tight rubber stopper secured to the top was employed as the cell vessel. A porous sintered glass cylinder was inserted into the cell vessel through the rubber stopper to a point adjacent to the bottom of the cell vessel. The porous sintered glass served as a diphragm to separate the cathode section (the volume outside of the sintered glass cylinder), from the anode section (the volume inside the sintered glass cylinder).

A disk of Woods metal, which was about one-quarter inch thick and about three inches in diameter was positioned in the cathode section as the cathode. A threeeighths-inch diameter graphite rod, which was approximately six inches long, was inserted into the anode section to serve as the anode. Copper wire connected the graphite rod and the disk of Woods metal to a source of direct current. Glass tubing passedthrough the top of the cell vessel served as a means of removing catholyte gas, and glass tubing inserted in the gas-tight cover of the sintered glass cylinder served to remove theanolyte gas.

Thirty grams of molten phosphorus was placed in the cathode section of the cell vessel. Four hundred milliliters of a saturated aqueous solution of sodium chloride added to the cathode and anode sections to serve as an electrolyte. The disk cathode contacted the molten phosphorus as well as the electrolyte. Agitation of the electrolyte and molten phosphorus was effected by means of a plastic coated magnetic stirrer which was rotated at the rate of about two hundred revolutions per minute.

A current of one ampere and a voltage of eighteen volts were impressed upon the system during electrolysis. The temperature of the electrolyte and molten phosphorus was maintained at about eighty degrees centigrade by placing the cell vessel in a constant temperature bath.

The gaseous mixture of phosphine and hydrogen produced at the cathode had a phosphine concentration of fifty percent by volume and was produced at the rate of eight milliliters per minute. Substantially pure chlorine was produced at the anode at a corresponding rate. The catholyte contained sodium hypophosphite in a concentration of about five percent by weight.

Example 2 The procedure of Example 1 was repeated, employing the cell vessel of Example 1, with the following exceptions. A one molar aqueous lithium chloride solution was employed as the electrolyte in place of the sodium chloride solution. A current of one ampere and a voltage of eight volts were impressed upon the system during electrolysis.

The gaseous mixture of phosphine and hydrogen produced at the cathode had a phosphine concentration of about thirty-seven percent by volume, and was produced at the rate of about twelve milliliters per minute. Substantially pure chlorine was produced at the anode at a corresponding rate. The catholyte contained lithium hypophosphite in a concentration of about five percent by weight.

It will be recognized by those skilled in the art that various modifications within the invention are possible, some of which are referred to above. Therefore, I do not wish to be limited except as defined by the appended claims.

I claim:

1. A process for the production of phosphjne, a halo- .gen gas, and an alkali metal hypophosphite which comprises passing an electric current between an anode and a cathode in contact with an electrolyte comprised of an aqueous solution of an alkali metal halide, said cathode being in Contact with molten phosphorus, whereby phosphine is produced at the cathode, whereby a halogen gas is produced at the anode, and whereby an alkali metal hypophosphite is produced in the catholyte, and separately recovering the resulting phosphine, halogen and alkali metal hypophosphite products.

2. The process of claim 1 wherein said cathode is in agitated contact with said molten phosphorus.

3. The process of claim 1 wherein said cathode is an alloy of bismuth, lead, tin and cadmium.

4. The process of claim 1 wherein said cathode is mercury.

5. The process of claim 1 wherein said anode is graphite.

6. The process of claim 1 wherein said alkali metal halide is an alkali metal chloride and the resulting halogen gas is chlorine.

7. The process of claim 6 wherein said alkali metal chloride is sodium chloride and said alkali metal hypophosphite is sodium hypophosphite.

8. The process of claim 6 wherein said alkali metal chloride is lithium chloride and said alkali metal hypophosphite is lithium hypophosphite.

9. The process of claim 1 wherein the temperature of the electrolyte and molten phosphorus is maintained during electrolysis within the range between about forty-four and two hundred and eighty degrees centigrade.

10. The process of claim 1 wherein the temperature of the electrolyte and molten phosphorus is maintained during electrolysis within the range betwen about fifty and about one hundred and twenty degrees centigr-ade.

11. The process of claim 1 wherein the current density of said cathode during electrolysis is maintained in the range between about five and one thousand amperes per square foot.

12. The process of claim 11 wherein the voltage during electrolysis is maintained below about twenty volts.

13. A process for preparing phosphine, chlorine, and an alkali metal hypophosphite which comprises passing an electric current between a graphite anode and a cathode in contact with an electrolyte comprised of an aqueous solution of an alkali metal chloride, a portion of said electrolyte in contact with said cathode being admixed with molten phosphorus, said molten phosphorus being in agitated contact with said cathode, and maintaining a current density on said cathode of "at least about five amperes per square foot, whereby phosphine is produced at the cathode, whereby chlorine is produced at the anode, and whereby an alkali metal hypophosphite is produced in the cathode, and separately recovering phosphine, chlorine and alkali metal hypophosphite products.

14. The process of claim 13 wherein said alkali metal chloride is sodium chloride and said alkali metal hypophosphite is sodium hypophosphite.

15. The process of claim 13 wherein the said alkali metal chloride is lithium chloride and said alkali metal hypophosphite is lithium hypophosphite.

16. The process of claim 13 wherein said alkali metal chloride is potassium chloride and said alkali metal hypophosphi-te is potassium hypophosphite.

References Cited in the file of this patent UNITED STATES PATENTS 1,375,819 Blumenberg Apr. 26, 1921 1,970,973 Palmaer Aug. 21, 1934 2,867,568 Cunningham Jan. 6, 1959 FOREIGN PATENTS 1,130,548 France Oct. 1, 1956 

1. A PROCESS FOR THE PRODUCTION OF PHOSPHINE, A HALOGEN GAS, AND AN ALKALI METAL HYPOPHOSPHITE WHICH COMPRISES PASSING AN ELECTRIC CURRENT BETWEEN AN ANODE AND A CATHODE IN CONTACT WITH AN ELECTROLYTE COMPRISED OF AN AQUEOUS SOLUTION OF AN ALKALI METAL HALIDE, SAID CATHODE BEING IN CONTACT WITH MOLTEN PHOSPHORUS, WHEREBY PHOSPHINE IS PRODUCED AT THE CATHODE, WHEREBY A HALOGEN GAS IS PRODUCED AT THE ANODE, AND WHEREBY AN ALKALI METAL HYPOPHOSPHITE IS PRODUCED IN THE CATHOLYTE, AND SEP- 